Local area networks (LANs) allow organizations to share information over a high speed network that may be assembled with relatively inexpensive hardware components. Until recently, LANs were limited to hardwired infrastructure, requiring the user to physically connect to the LAN via a wired connection. However, with the recent growth of wireless telephony and wireless messaging, wireless communications have also been applied to the realm of LANs, resulting in the development of wireless local area networks (WLANs). Like typical LANs, WLAN systems also provide high performance with relatively inexpensive hardware components at a low cost point. One of the biggest challenges in designing a low cost WLAN communication system is designing a WLAN receiver that accurately matches the frequency of the WLAN receiver to a WLAN transmitter.
IEEE 802.11a specifies an over-the-air interface between WLAN receivers and WLAN transmitters so that communications can take place in spite of the challenge of accurately matching the frequency of the WLAN receiver to the WLAN transmitter. Specifically, IEEE 802.11a specifies that at 5 GHz, with data speeds of up to 54 Mbps where each data channel is 20 MHz, a crystal may be utilized in the WLAN receiver and the WLAN transmitter so that a tolerance of 20 ppm is met. Further, the standard provides for the use of a digital frequency corrector to compensate for this error because this error generally increases over time and can approach 40 ppm. The digital frequency corrector takes a frequency estimate generated by a preamble and training sequence block to correct a received signal. The problem with the IEEE 802.11a solution is that the frequency estimate provided by the digital frequency corrector is imperfect and over time the digital frequency corrector causes the transmission channel to become quite noisy. As a result, the degradation in the quality of the frequency estimate contributes to performance degradation of the WLAN communication system.
The problem is more acute where narrower channels are used. For example, a 4.9 GHz Mission Critical Local Broadband (MCLB) system specifies channels of bandwidth 5 MHz, a proposed IEEE standard 802.11j specifies channels of bandwidth 10 MHz, and a 5.9 MHz Digital Short Range Communication (DSRC) system requires channels of bandwidth 10 MHz. Narrower channels necessarily require more accurate crystals for communications to take place and more accurate crystals currently cost more. For example, for a 5 MHz channel, a crystal with a tolerance of 10 ppm is required. The requirement of low cost is inapposite to the requirement of accurate crystals.
A further problem with more accurate crystals, such as crystals below 10 ppm, is that they require thermal stabilization and call for significant power requirements. Such requirements typically are cost prohibitive and/or difficult to meet by the hardware currently available for WLAN receivers and WLAN transmitters.
While the existing method of providing frequency correction in a WLAN communication system is relatively satisfactory, overtime the method causes performance degradation of the WLAN communication system. Accordingly, there exists a need for a better method and apparatus that permits frequency correction in a WLAN communication system.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.